In the manufacturing of papermaking pulp, the pulp is, inter alia, subject to bleaching using bleaching chemicals. A widely used bleaching chemical is chlorine dioxide (ClO2). Chlorine dioxide is often produced on site at the pulp mill that uses the chlorine dioxide in its pulp bleaching. There are various ways of producing chlorine dioxide. One such generally applied process is the Mathieson process, which was developed in the 1950s by Olin-Mathieson Chemical Corporation. In this process, chlorine dioxide is formed by reduction of sodium chlorate (NaClO3) with sulphur dioxide (SO2) in the presence of sulphuric acid (H2SO4). Another common process for producing chlorine dioxide is a process resembling the Mathieson process, but using hydrogen peroxide (H2O2) instead of sulphur dioxide. In this process, hydrogen peroxide and sodium chlorate are caused to react in the presence of sulphuric acid to form chlorine dioxide.
In both processes described above not only chlorine dioxide forms, but also a residual product, which mainly consists of sulphuric acid and sodium hydrogen sulphate (NaHSO4). This residual product is generally called “residual acid”. Sometimes the residual acid can to some extent be used by the pulp mill as an agent for adjusting the pH or as an agent for cleaving tall oil. But the residual acid is mainly a waste product. Before disposal, this acid waste product has to be neutralized. Considering that a typical pulp mill produces about 10,000 tonnes of residual acid a year, it is obvious that great efforts and costs are required to render the residual acid nonhazardous and to dispose of it.
Therefore it would signify a great step forward in this technical field if the residual acid could be used as a starting material for the production of useful and valuable products, instead of being disposed of at high costs.